In the ongoing information technology revolution, display elements and assemblies thereof, i.e., monitors such as displays, play an important role as interfaces between people and various types of equipment. Such display elements are faced with continuous demands for higher brightness and higher resolution, and must also be as thin and lightweight as possible. Fluorescent materials with high fluorescence quantum yield and brightness are indispensable for meeting such requirements. Moreover, such fluorescent materials are also used in a wide variety of applications for lighting materials. Therefore, research has been conducted to develop such fluorescent materials for about 100 years, starting in the beginning of the 20th century.
Dyes and metal ions have long been known as fluorescent materials. As fluorescent materials for use in display elements and lighting devices, inorganic matrices in which metal ions, particularly rare earth ions or transition metal ions, are dispersed have so far been used. These fluorescent materials have been researched extensively and are being constantly improved because rare earth ions and transition metal ions change little over time and are degraded only minimally by light irradiation in comparison with organic Dyes. However, the transition of such rare earth ions and transition metal ions is a forbidden transition type in many cases, and hence their emission lifetime is about 1 ms (millisecond). Accordingly, even when such ions are irradiated with intense excitation light in an attempt to achieve higher brightness, such ions fail to rapidly convert this light to the desired emitting light, and the phenomenon referred to as brightness saturation occurs. This phenomenon is a major obstacle to achieving higher brightness through the use of rare earth ions or transition metal ions. Moreover, it is generally necessary to vary the excitation wavelength according to the wavelength of the emitted light.
For the last ten years, research has progressed on the fluorescence properties of materials obtained by doping the above-described transition metal ions or rare-earth ions into ultrafine particles. However, the emission lifetime of the ions doped in the ultrafine particles is not different from that of ions doped in a bulk matrix. Therefore, these materials are regarded as an extension of the above-described conventional fluorescent materials.
In recent years, attention has been drawn to the discovery that surface-treated semiconductor ultrafine particles (in which transition metal ions or rare earth ions are not doped) emit light with high efficiency. Typical examples of semiconductor ultrafine particles are particles of Group II-VI compounds whose diameters are on the order of several nm. These particles display so-called quantum size effects, with smaller particles having a wider band gap. For this reason, such particles can emit various colors depending on the particle size even when all are irradiated with ultraviolet light of the same wavelength, with smaller particles emitting shorter wavelengths of light. For example, when irradiated with ultraviolet light, cadmium selenide emits blue light when in the form of particles with a diameter of about 2 nm, and red light when in the form of particles with a diameter of about 5 nm. The emission lifetime of these semiconductor ultrafine particles is about 10 ns (nanosecond). For this reason, excitation light can be converted to the necessary light at a speed five orders of magnitude more rapidly than with a fluorescent material based on rare earth ions or transition metal ions, and the excitation light can be reabsorbed and emitted, making it possible to ensure extremely high brightness. Another advantage is the ability to promptly follow the ON and OFF cycles of excitation light.
It should be noted, however, that such semiconductor ultrafine particles have a large specific surface area because of their small particle size, which makes it important to reduce the number of surface defects by a surface treatment to suppress radiationless deactivation in order to raise the fluorescence quantum yield. Sulfur-containing compounds can suitably be used for such surface treatment. Typical examples include thiols and other organic surfactants, as well as zinc sulfide. Since semiconductor ultrafine particles whose surface is thoroughly coated using such compounds are incredibly bright, it has been shown in the latest research that emission from each individual particle can be separately detected and spectroscopically resolved. This can be regarded as an excellent characteristic unattainable with a rare earth or transition metal fluorescent material. Another significant advantage of semiconductor ultrafine particles is that various colors can be emitted in accordance with the particle size by irradiating the particles with light whose single wavelength is shorter, that is, has higher energy, than the band gap. In other words, the advantage of such fluorescent materials is that the excitation wavelength can be suitably selected and that even when the same material is used, emission with the desired wavelength can be obtained by varying the particle size.
Such semiconductor ultrafine particles are currently produced by a colloidal method. There are two types of particles: those that are produced in an aqueous solution, and those that are produced in a nonaqueous solution.
A typical example of such ultrafine particles produced in an aqueous solution is cadmium telluride, which has a fluorescence quantum yield of several percent (Gao, et al., Journal of Physical Chemistry, B, vol. 102, p. 8360 (1998)). Ultrafine particles produced by this procedure are dispersed in water and are stable for a while, however, they are inferior in the fluorescence quantum yield to the ultrafine particles produced in a non-aqueous solvent described below.
In contrast, it was recently reported that ultrafine particles produced by an aqueous solution method were subjected, after the formation of ultrafine particles, to a process of decreasing the pH of the solution or to a process of etching the particles by irradiating them with light, thereby manufacturing ultrafine particles with a fluorescence quantum yield of about 40% (Gaponik et al., Journal of Physical Chemistry, B, vol. 106, p. 7177 (2002)). However, ultrafine particles produced in a pH-reduced solution are unstable, and fluorescence quantum yield decreases to half or less in air in about 7 days. Since etching ultrafine particles by irradiation with light requires about five days and the particle size distribution of the produced particles expands, the width of the emission spectrum also adversely enlarges.
A process by which ultrafine particles are produced using organometallic pyrolysis is known for producing such fluorescent materials in a nonaqueous solution (Bawendi, et al., Journal of Physical Chemistry, B, vol. 101, p. 9463 (1997)). This method is advantageous in that, for example, a quantum yield in excess of 20% can be obtained with ultrafine particles of cadmium selenide, and although the resulting ultrafine particles themselves are insoluble in water, coating the surface with ionic organic molecules makes the particles soluble in water so they can be handled in the same way as the previously described cadmium telluride particles obtained from an aqueous solution. However, this method disadvantageously requires expensive and complicated experimental equipment, and special considerations to ensure safety. Another drawback is that in most cases ultrafine particles thus produced remain stable in water for only about a few hours. Therefore, these drawbacks are major factors impeding the practical application of particles produced in a nonaqueous solvent.
Thus, surface-coated semiconductor ultrafine particles emit extremely bright light but cannot easily be kept stable in an aqueous solution. Ultrafine particles of cadmium telluride obtained by an aqueous solution method usually aggregate and precipitate in about five days in air at room temperature. For this reason, the emission performance is lost.
For the above reasons, conventional semiconductor ultrafine particles lack practicality as an engineering material because the particles in the form of a solution cannot maintain their fluorescence quantum yield at room temperature in air for a long time, even if the particles originally emit extremely bright light.
Several attempts have therefore been made to support and stabilize such surface-coated semiconductor ultrafine particles in a solid matrix. There is, for example, a report concerning a method for fixing such particles in an organic polymer (Bawendi, et al., Advanced Materials, vol. 12, p. 1102 (2000)). However, polymers used as a matrix have low levels of light resistance, heat resistance, and other properties, and gradually permit the passage of water and oxygen. The resulting drawback is a gradual degradation of the ultrafine particles thus fixed. In addition, in a mixture of ultrafine particles as an inorganic material and a polymer as an organic material, the ultrafine particles are apt to aggregate if the dispersion concentration of the ultrafine particles is high, and therefore the fluorescent material tends to have inferior characteristics as a light-emitting material.
To overcome the drawbacks of such polymer matrices, an attempt has been made to disperse ultrafine particles in a glass matrix by a sol-gel process using tetraalkoxysilane (Selvan, et al., Advanced Materials, vol. 13, p. 985 (2001)). In this method, however, the ultrafine particles are insoluble in water, so usable sol-gel processes are limited and only gelled products can be obtained. In addition, the fluorescence quantum yield in this case is reported to be about 10% at the maximum.